Another phrasing problem with the question - the maximum size of an index is not 900 bytes. The maximum size of the key columns is 900 bytes. The index row can technically be up to 8000 bytes.
Although what you say is correct, your wording also seems misleading.
The actual limitation is listed in the reference material as:
Index key columns, excluding nonkeys, must follow the existing index size restrictions of 16 key columns maximum, and a total index key size of 900 bytes.